A method and gasmeter of the above identified type are disclosed by WO 9410540. When the flow rate of gas passing along the first sensor decreases or increases the first sensor will cool down slower, faster respectively after being heated up. Therefore, since a current cooling rate value can be measured, a corresponding value of the current flow rate can be derived from the current cooling rate value by using said calibration table.
The density of the gas may change by changes of temperature and/or pressure of the gas. When the density increases or decreases a larger, smaller amount of gas molecules will strike the sensor per unit of time and therefore the sensor will cool down faster, slower respectively. Accordingly, this will be reflected to the current flow rate value too derived from the measured current cooling rate value. In other words, when referring to a situation with a reference temperature and reference pressure, the gasmeter will determine the gas flow rate in terms of a quantity of gas molecules per unit of time, rather than a volume per unit of time (as with commonly used bellow meters).
When the gasmeter has been calibrated with said reference temperature and said reference pressure (or different references derived therefrom by using the law of Boyle-Gay-Lussac), a quantity of gas molecules passed per unit of time, represented by a measured flow rate value, is associated with a current cooling rate value as calibration value. When measuring an identical cooling rate value afterwards, the current flow rate value will be identical to the corresponding calibration flow rate value also, no matter the values of the temperature and pressure at that later time. Also, when the same quantity of gas molecules as during calibration passes during identical periods of time during calibration and thereafter, the gasmeter will provide identical flow rate values, no matter the values of the temperature and pressure at the later time. Therefore the gasmeter can be calibrated to provide a current flow rate value as volume of time still.
A drawback of the prior art gasmeter is that, when in actual use, the provided flow rate values will differ when a current gas is different from a gas used during calibration. Therefore it will be necessary to calibrate the gasmeter by using a gas which is identical to a gas for which the gasmeter is destined to be used with. In many situations, such as in case of using natural gas, this may be a problem in view of safety, waste gas handling and costs. To provide a gasmeter of this type with a certain measurement accuracy range, this requires the manufacturing, calibration and keeping in stock of as many types of gasmeters, resulting in a further increase of costs.
In addition, after installation, the density of the gas to be measured may differ for other reasons. For example, in case of natural gas, the gas suppliers receive gas of several compositions and they will try to provide a mixture thereof to consumers having a heating power per unit of volume which is as constant as possible. To do so any suitable auxiliary gas, such as nitrogen, may be added. However, the result of this will be that the density of the resulting gas (or gas composition) supplied to the consumers may differ from time to time for identical temperatures and pressures resulting in different cooling rate values and therefore different associated flow rate values in spite of identical of such volumes per unit of time and therefore leading to incorrect measurement values.
U.S. Pat. No. 4,885,938 discloses a method for compensating the mass flow measurement of a fluid flowmeter of the thermal microananometer class for changes in the composition of the fluid of interest the flow of which is thought to be determined or monitored. The method comprises the steps of: obtaining an on-going uncorrected nulled mass flow value for the fluid of interest in relation to the microananometer sensor output; obtaining the specific heat thermal conductivity and density of the fluid; and obtaining the corrected mass flow from the nulled mass flow according to a specific formula containing said four independent variables. To that end values of the specific heat, thermal conductivity and density of the gas are derived from a static ananometer measurement of the fluid of interest by using a chamber which communicates with the proper or main fluid channel, in which the chamber provides basically a static environment with respect to flow. The channel contains a first microbridge or microananometer sensor and the chamber contains a second sensor of the same type as of the first sensor. Such type of microbridge sensors must be mounted with a specific orientation with respect to a flow of fluid passing along it, one branch of the bridge being heated above ambient temperature and an unbalance of the bridge being measured to determine the flow rate of the passing fluid. The flow rate determined by the first sensor is nulled, i.e. it is corrected by subtracting its value obtained at zero flow. The second sensor is used to determine values for the specific heat, thermal conductivity and density of the fluid contained basically static in the chamber. The document does not disclose how this is carried out but refers to a system described in co-pending applications thereof.
Though the formula disclosed by U.S. Pat. No. 4,885,938 is simple, its application requires an additional system for deriving values of three independent variables, which makes this prior art method and flow meter too complex, bulky and expensive for use in a domestic gasmeter.
While the method disclosed by U.S. Pat. No. 4,885,938 measures and compensates a flow rate value directly, with the method disclosed by WO9410540 a cooling rate of a preheated sensor is measured without measuring or determining other properties of the current fluid after installation of the meter.